1. Field of the Invention
The present invention refers to a monolithic circuit of active quenching and active reset for avalanche photodiodes.
2. Related Art and Other Considerations
The semiconductor detectors of single photons (Single Photon Avalanche Diode, SPAD) are special avalanche photodiodes operating in Geiger mode, that is at a bias voltage higher than the breakdown voltage, so that it is possible to detect the single photons.
These single photon avalanche diodes represent a definite progress compared to the photomultiplier tubes (PMT) in measurements made by counting the incident photons, thanks to their greater compactness, reliability and ruggedness, to their small dimensions, to the lower bias voltage and thanks to their high detection efficiency in the infrared spectral region (IR) and in the near infrared region (NIR).
The SPAD""s are, essentially, p-n junctions capable of being biased at a voltage Va, higher than the breakdown voltage Vbd. At this bias voltage the electric field is so high that the absorption of a single photon can trigger a phenomenon of self-sustainining avalanche multiplication, capable of producing a current signal (of the order of milliamperes) which can be easily distinguished from the noise of the electronic circuits.
The operating principle of the SPAD is conceptually different from that of the usual avalanche photodiodes, biased below the breakdown voltage. In the latter ones, the avalanche multiplication process produces an internal gain that acts like an amplifier of the current generated by the absorption of a photon.
The SPAD behaves like a bistable-type switching circuit. In fact, once the avalanche is triggered, the current continues to flow until the avalanche is quenched by means of lowering the bias voltage below the level of the breakdown voltage. The bias voltage is brought back above the breakdown voltage, to the preset operative level Va, so that another incident photon can be detected. This operation requires a suitable circuit, which is normally defined as quenching circuit.
The known technique proposes as method suitable for obtaining this result a so-called Passive Quenching Circuit (PQC), in which a high value resistance RL (hundreds of KOhm) is inserted in series to the bias voltage generator. Once the avalanche has been started, the current rapidly discharges the capacity of the device, forcing the voltage at the leads of the diode to drop down to a value near to the breakdown voltage. If the RL is sufficiently high, the final current results to be lower than the limit current (of the order of hundreds of microamperes) below which the avalanche is no longer capable of sustaining itself. After the avalanche quenching, the voltage at the leads of the diode is brought back to the bias voltage, but at a much slower rate because the capacity of the diode is charged by the small current that flows through the resistance RL. The reset of the bias voltage follows an exponential law with a time constant that results to be about 1 microsecond or more.
During this slow transient the SPAD can be triggered by another incident photon while it is at an intermediate bias voltage between the breakdown voltage and the preset operative voltage Va. As the detection efficiency of the photons increases with the excess bias voltage Vexc, defined as the difference between the operative bias voltage Va and the breakdown voltage Vbd, during the recovery times there can be counting losses that are difficult to quantify. All this causes a non-linearity in counting the photon number which, the greater the counting rate is, the more marked it becomes.
In fact, the use of PQC circuits is limited to the applications in which the average counting rate does not exceed some thousand counts per second (Kcounts/s).
In order to exceed the limits of the passive quenching circuits, the active quenching circuits AQC have been introduced.
The operating principle of the AQC demands that the leading edge of the avalanche current be detected by a comparator, which produces a standard output pulse synchronized with said edge. The same edge activates a controlled generator that lowers the bias voltage of the SPAD below the level of the breakdown voltage, thus quenching the avalanche. The voltage at the leads of the photodiode is kept low for a preset time interval, at the end of which the bias voltage is rapidly brought back to the operative level Va.
The transition that brings back the bias voltage to the operative level Va must be rapid, to reduce to a minimum the interval of time in which the voltage has not yet reached the preset value Va, but is nevertheless above the level of the breakdown voltage Vbd.
In comparison to the PQC circuits, the AQC have two fundamental advantages: a) the avalanche is always triggered in standard conditions of bias, thus guaranteeing a well-controlled detection efficiency; b) the dead time in which the detector is insensitive is well defined by the user, thus permitting an accurate correction of the data.
Nevertheless the AQC circuits proposed up to now have various drawbacks: the dimensions are not miniaturized, as the circuit schemes are not suitable for implementation in monolithic integrated form; the reliability in applications with harsh environment is insufficient, as the circuits are assembled using discrete components and/or various integrated circuit blocks; the power dissipation is significant even in stand-by conditions, which involves further limitations to miniaturization and to use in portable apparatus.
In view of the state of the technique described, the object of the present invention is to implement a new AQC circuit having a circuit structure such that makes possible its integration in conventional CMOS technology.
In accordance with the present invention, this object is reached by means of a monolithic circuit of active quenching and active reset for avalanche photodiodes detecting single photons (SPAD), comprising quenching means which are sensitive to the triggering of the avalanche and reset means operating with a preset time delay compared to said quenching means and for a preset length of time, characterized in that said quenching means comprise a first couple of transistors of opposite polarity connected so that they implement a positive feedback action capable of lowering the bias voltage of the photodiode to below the breakdown voltage at every triggering of the avalanche.
In addition, the reset means preferably comprise a second couple of transistors activated with a preset delay and connected so that said positive feedback is interrupted for a preset time duration and the initial bias voltage is reset at the leads of the photodiode.
Thanks to the present invention an AQC circuit can be made with similar dimensions to the SPAD detector so that the entire structure can be integrated on a single chip, the power dissipation can be reduced because there are no circuits operating in linear regime and the costs are reduced, at the same time keeping good performance.
The compactness of the AQC thus achieved enables to fabricate miniaturized multichip modules for photon counting, that can be housed in a single case, for example a standard TO-8 case, that can also include a Peltier cell for controlling the temperature.
In addition, given the reduced dimensions, the photon-counting module can be replicated for employing it in multidetector systems and SPAD matrix systems.
Furthermore, the low power dissipation makes possible to use these circuits in battery-powered systems and is an essential feature for the applications wherein the generation of heat influences the measurements.